1. Field of the Invention
The present invention concerns a method and an apparatus for generating a digital x-ray image of an examination subject.
2. Description of the Prior Art
In the acquisition of an x-ray image of an examination subject, for example in mammography, it must be ensured that the acquisition parameters (in particular the exposure time) are set correctly in order to achieve an image quality suitable for diagnostic evaluation. In order to limit the radiation exposure of the irradiated region of the examination subject to the diagnostically-required minimum, it is intended to already correctly set the acquisition parameters in the first acquisition. For this purpose, in the prior art an automatic exposure (AEC, automatic exposure control) is used. For mammography, a number of solid-state detectors are arranged behind the x-ray film (viewed in the propagation direction of the x-rays) that measure the intensity of the x-rays transmitted through the x-ray film. The output signal is used to control the acquisition parameters (for example exposure time, operating voltage of the x-ray tube, tube current, anode filter combination).
Due to the higher absorption of the solid-state detector array used in the acquisition of digital x-ray images, such a procedure is not possible for the acquisition of digital x-ray images. In digital x-ray acquisition, the acquisition parameters suitable for control of the exposure are determined in a pre-shot with a reduced radiation dose, in which a number of individual detectors of the matrix-like solid-state detector array are respectively combined into a measurement field and an average intensity within this measurement field is determined. In this manner, a number of measurement fields (for example 600 measurement fields) are generated in which the average brightness is respectively determined. The acquisition parameters (in particular the exposure time for the following acquisition of the digital x-ray image) are derived from these 600 average values. Due to this averaging, the measurement data acquired in the pre-shot cannot be diagnostically utilized and thus represent an additional radiation exposure for the patient.